Osmium is named, almost uniquely, for a smell. Its tetroxide is volatile and intensely irritating, with a penetrating chlorine-like odour that gave the element its Greek name, osme; the same compound is a hazard to the eyes and lungs and a useful stain in microscopy. The element has no mythological or alchemical tradition, being a nineteenth-century discovery among the platinum metals, but it holds a physical distinction that lends it a quiet fame: it is the densest substance the Earth naturally provides, so that a fist-sized lump is startlingly, almost unbelievably heavy. Discovered in the same platinum residues by the same chemist as iridium, osmium is the pungent, ponderous twin of the rainbow-named element.
Like its platinum-group neighbour iridium, osmium is a siderophile element that sank overwhelmingly into the Earth's molten core as the planet formed, leaving the crust unusually depleted in it relative to meteorites and asteroids; geochemists accordingly use osmium isotope ratios, alongside iridium, as a further chemical fingerprint for identifying ancient asteroid and comet impacts in the geological record. The two elements, discovered by the same chemist in the same platinum residue, thus ended up telling much the same story about catastrophe from space more than a century and a half later.